The present invention relates to a misfire detecting system for a multi-cylinder internal combustion engine for detecting misfiring on the basis of variation of an engine speed.
Conventionally, a system for detecting misfiring of an internal combustion engine has been known, in which a rotational speed of a crankshaft before and after an expansion stroke of a reciprocal internal combustion engine is detected and misfiring is judged when a rotational speed difference is equal to or less than a set value, as disclosed in JP-A-58-19532. Namely, the rotational speed of the crankshaft differs between an expansion stroke and a compression stroke, and the rotational speed in an expansion stroke is higher than that in a compression stroke when spark ignition takes place normally. By contrast, when misfiring occurs, a difference in the rotational speed between an expansion stroke and a compression stroke is reduced and the rotational speed becomes substantially equal therebetween. On the basis of this fact, misfiring is judged when the rotational speed difference is equal to or less that a predetermined value, namely, when the crankshaft is not accelerated during the expansion stroke.
On the other hand, there is another system for detecting misfiring of the internal combustion engine, in which a predetermined number of engine speed variations are sampled, and a standard deviation is calculated on the basis of the sampled engine speed variations to thereby detect occurrence of misfiring when the standard deviation is large, namely, when the dispersion of the engine speed variations is large (for example, JP-A-58-51243).
However, in the system set forth above, it is possible to make an erroneous determination of misfiring despite the fact that misfiring is not occurring when a vehicle travels on a rough road due to unstable rotation of the crankshaft caused by roughness of the road surface. Namely, on a rough road, a road/tire traction is not uniform, rather it varies significantly due to the roughness of the road surface, as the vehicle bounds and rebounds on the road surface. Variations of the road/tire traction causes fluctuation of a load on the internal combustion engine. For instance, when a load on the engine is increased, the rotational speed of the crankshaft cannot be increased even when the engine is in an expansion stroke where the crankshaft rotational speed should be increased under a normal operating condition. If the crankshaft rotational speed is not increased in an expansion stroke where the crankshaft rotational speed should be increased, variations of the crankshaft rotational speed becomes larger, resulting in larger dispersion of the rotational speed variations. This eventually poses a problem in that the detection of the occurrence of misfiring cannot be achieved accurately.
Here, the transition state of a variation magnitude .DELTA..omega. of engine speed variations upon the occurrence of one time misfiring when a vehicle is travelling on a smooth road and the transition state of a variation magnitude .DELTA..omega. of engine speed variations when the vehicle is travelling on a rough road differ from each other, as will be discussed with reference to FIGS. 15A and 15B. In the former case, the ratios of .DELTA..omega. at the time of occurrence of misfiring to .DELTA..omega.'s before and after the occurrence of misfiring are both large. By contrast, in the latter case, the ratios of .DELTA..omega. when engine speed variations are caused to occur due to roughness of the road surface to .DELTA..omega.'s before and after the occurrence of the engine speed variations are both small. Accordingly, by checking the magnitude relation of the two ratios, satisfactory accuracy in detecting the occurrence of misfiring can be obtained even when the vehicle is travelling on a rough road.
However, if a case continues where misfiring occurs in a plurality of successively ignited cylinders when checking the magnitude relation of the ratios of .DELTA..omega. at certain timing with respect to .DELTA..omega.'s before and after that timing, such successive occurrence of misfiring cannot be detected. It is because, if the case continues where misfiring occurs in both of two successively ignited cylinders, for example, the transition state of .DELTA..omega. becomes as illustrated in FIG. 15C. In this case, among two ratios of .DELTA..omega. at the occurrence of misfiring to .DELTA..omega.'s before and after the occurrence of misfiring, one ratio becomes small, while the other ratio becomes large. Therefore, misfiring cannot be detected, since both of the two ratios of .DELTA..omega. are not large.
Furthermore, erroneous determination of misfiring may be caused by juddering of a vehicle following the occurrence of one time misfiring. Here, the juddering of a vehicle is a kind of hunting phenomenon repeating such motions that, when the engine speed variations are reduced by the occurrence of misfiring, the engine is driven by the inertia of the vehicle, as a result, the engine speed is increased as a reaction of a drop it, the engine speed variations, then, as a reaction of a resultant excessive rise of the engine speed, the engine speed decreases again, as shown in FIGS. 10A-10D. This phenomenon is attenuated and the engine speed is stabilized at a certain speed. This juddering is caused only following a final misfire in the case of successive occurrence of misfiring. Once juddering is caused, the engine speed variation magnitude increases during a certain period following the occurrence of misfiring with a possibility of leading to erroneous determination of misfiring.